McStay began his senior career when he joined
Celtic from
Larkhall Thistle in 1912, although he was immediately loaned to
Ayr United, spending the following four seasons with the
Somerset Park club. Having helped Celtic to the
Scottish Football League title in
1917, He had further spells away from
Celtic Park during the latter years of the
First World War, assisting local sides
Distillery and
Belfast Celtic while conscripted to the
North Irish Horse unit, before re-establishing himself in the Celtic first team which won the title again
1919 and
1922. McStay was suspended by Celtic following a wages dispute in the aftermath of their
1923 Scottish Cup Final victory, and he spent the summer in the United States; he played one game with
New York Field Club in the
American Soccer League. He was appointed
captain of Celtic upon his return to the club later that year, taking over from
William Cringan, and held the role for six years. During this time he won one further League Championship (in
1926) and two more
Scottish Cups. Taking all cups into account, he played in 470 matches for the club. and retired from playing in 1930. He was later manager of
Glentoran for a period in the mid-1930s. ==International career==